1. Field of the Invention
The present invention relates to a process for manufacturing thermoplastic resin moldings in a short molding cycle. The resin moldings are nearly free from resin warp, exhibit minimal deformation, and have good appearance.
2. Related Background Art
Thermoplastic resin moldings can be used in various applications including interior parts of automobiles (for example, door trims, rear trims, instrument panels), exterior and interior parts of household appliances, sundries, and the like. Thermoplastic resin moldings are typically manufactured by injection molding or press molding.
If desired, the surfaces of thermoplastic resin moldings can have a skin material entirely or partly laminated thereon. Entirely or partly laminated means that, respectively, the surface of the molding is entirely or partly covered with an integrally laminated skin material.
For injection molding processes, molten and plasticized thermoplastic resin (hereinafter sometimes referred to simply as "resin") is filled via a resin feed gate in a hermetically sealed mold cavity at high pressure. This generates a so-called oriented resin and may result in molding deformation, such as a twist or warp in the thermoplastic molding. The warp of resin can be particularly pronounced in the vicinity of the feed gate. Twisting or warping is disadvantageous.
For conventional press molding processes, molten resin is fed between open opposing male and female molds, is allowed to uniformly run on at least one of the mold surfaces, and is shaped by clamping together the male and female molds. The thermoplastic resin molding is nearly free from warp, and deformation is scarcely generated in the resultant thermoplastic resin moldings. However, because the molten thermoplastic resin is generally fed through a plurality of resin feeding gates, the obtained moldings have conspicuous weld-lines on their surfaces and a defective appearance, as well as strength problems. Raising the mold temperature during molding can reduce the generation of weld-lines. This means, however, that it will take a long time to cool and solidify the molten resin, resulting in a relatively long molding cycle.